A voltage regulator is an electrical regulator designed to automatically maintain a constant voltage level. A voltage regulator takes an environmentally sensitive voltage input and generates a stable output voltage. Therefore, constructing a voltage regular with an accurately regulated output voltage over various environments (e.g. process, voltage, and temperature, “PVT”) is an important design goal. Precise control of load regulation (ΔVOUT/ΔI) across various environments is desirable. For example, in embedded Dynamic Random Access Memory (eDRAM) devices, accurate bit-line reference voltage is required for precise operations. In general, a voltage regulator should be able to isolate impacts of both source (supply) and sink charge from the regulated output and overcome over-voltage (or “hiccup”) problem. Further, a voltage regulator should be able to overcome instability at low output currents.
Another important characteristic of a voltage regulator is a high power supply rejection ratio (PSRR), which is used to describe the amount of noise from a power supply that can be rejected. The value of PSRR depends on the power supply being considered, thus the PSRR for the higher supply voltage is different than the PSRR of the lower supply voltage. The PSRR is defined as the ratio of the change in power supply voltage (ΔVDD) to the change in output voltage (ΔVOUT) caused by the change in the power supply,PSRR=ΔVDD/ΔVOUT. 
An ideal voltage regulator would have infinite PSRR. A real voltage regulator would have a finite PSRR, but a higher PSRR across all frequency (especially around chip resonance frequency of about a few MHz to 100 MHz) is desirable. Higher PSRR is crucial in many modern System on Chip (SOC) or System in Package (SiP) designs where power supply noise immunity is very important.
There are prior arts for producing voltage regulators, but some encounter over voltage (or “hiccup”) problem and/or low-load current instability problem. Some have improved on those problems but have not accurately regulated output voltage and/or have poor PSRR.
FIG. 1A illustrates an example of a conventional voltage regulator circuit. The circuit 100 suffers from over-voltage (or hiccup) problem. When load current rapidly changes from a large value to a very small value, the feedback loop 102 locked up because the feedback control loop 102 cannot remove the excess charge from the output capacitor 101 fast enough. Also, when the output current level is below a minimum required output current value, there is instability at the output voltage because the feedback loop 102 cannot function as designed due to insufficient gain of the loop. (The loop gain is proportional to the transconductance gm and output resistance Rout, and gm is proportional to the drain current through the PMOS 104.) Also, the circuit has a poor PSRR at mid to high power supply noise frequency, typically around a few MHz to 100 MHz, because the feedback loop 102 cannot respond fast enough for output (VREG) change resulting from power supply noise. The circuit response is slow due to the limited bandwidth and cannot respond to high frequency noise. Further, there is no trimming capability for high accuracy VREG output in the circuit 100 shown in FIG. 1A.
FIG. 1B illustrates the output current-voltage plot of an example conventional voltage regulator circuit as shown in FIG. 1A. As the current changes from zero to a small value (minus value means the current flows out), the output voltage changes rapidly until the output voltage reaches saturation (slower change), mainly due to the operation point of the PMOS transistor 104. Therefore, the prior art circuit suffers from a steep output voltage variation as shown in FIG. 1B. For example, an output voltage variation of about 30 mV was estimated from a conventional circuit similar to the circuit 100 shown in FIG. 1A. Further, a poor phase margin (less than 50 degrees) from the conventional circuit 100 can require a large decoupling capacitance of around 200 pF.
Accordingly, new methods and circuits for a voltage regulator to improve accuracy and overcome over-voltage problem, instability at low output currents, and poor PSRR are desired.